I am working on this arm in Zbrush, and it sudddenly occured to me that I might not want to have it be perfectly straight when I go to mount it on my guy. Is there a way, other than exporting it as an OBJ to a 3D program and modifying it there, to bend the arm at the elbow?
Lobo 
There may be other ways but only two options come to mind right now and they probably won’t give you really spectacular results. First you could mask off the shoulder and biceps all the way down to the elbow and use the MOVE tool to try to bend the rest of the arm. Second you can go into the deformations menu and OFFSET the image in the appropriate direction (X or Y most likely) then use the BEND command to bend it. It may take a little work to get it offset at just the right position. This will give you a very sharp edge where it was bent so it will have to be smoothed quite a bit. The best method would be to either remake the arm into the bent position from scratch OR make the upper arm and forearm out of two pieces so you can place them accordingly.
How do you al DO that?? This is incredible. I need to learn more on this program. Fantastic work. Hope to learn from you. And the rest of you. 
Well thanks, I just started with a sphere and moved it around till it was the rough shape of an arm, then I started with the details using the drawing tool.
This arm took about an hour, I have a head too but I think I’ll wait till I have most of the body done and together bofore I post too much more.
Yes I would say that Zbrush is a great tool if you don’t have time to learn a full 3D package, and having worked with Maya and XSI in the past I’d say that ZB’s sculpting tools are rival those in Maya. I have some samples of work I did in Maya which tool a lot long and looks about the same, http://www.aegisgraphics.com
And the advantage is that if you learn and intermediate modeling/texturing tool like ZB, then jumping to a full 3D platform is not too big of a leap.
Hola
in most of the projects i’ve done, i make the arm straight and then bend it using the “bend” and “sbend” in Tool - Modifiers - Deformation. i hope you don’t want the arm a whole lot, 'cause i can’t help you there.
anyway, make sure the elbow of your arm is in the center of the crosshairs in the little Tool - Modifiers window. then use the “bend” and “sbend” deformers to bend the arm. i hope this is clear enough to help you.
josh
Here is a little script showing what I was talking about. It is nothing fancy. Just turn the Replay Delay up to 1000 and make sure the Show Actions is selected.
The mask leaves kinda a nasty looking join in the mesh, I’ll work on it a tad more, maybe if I paint the mask at less than 100% opacity, <is that possible?> then it might deform a little nicer.
L
Jibberish’s SBend suggestion is probably the best bet.
In addition to making sure that the cross hairs are centered on where you want the bend to take place, remember that deformations are based upon the Object Preview window rather than anything on the canvas. So if you rotate the view in the Object Preview to where you are looking at the arm perfectly from the side, you’ll be able to restrict your bend to one axis and get exactly the result that you’re after.
well heres something i would try. use flatten to cut off the bottom part of arm. save that tool as TOPARM
open up the original tool and use flatten to cut off the top of the arm. SAve that as BOTTOMARM. then, you can further shape the pieces of the arm so they will fit to form a joint without starting all over.
edit: jibberishs method is still probably the best.